Secure SMTP/TLS SRV Announcement
draft-nurpmeso-smtp-tls-srv-04

Internet Engineering Task Force                         S. Nurpmeso, Ed.
Internet-Draft                                           3 February 2025
Updates: 3207 5321 (if approved)                                        
Intended status: Informational                                          
Expires: 7 August 2025

                    Secure SMTP/TLS SRV Announcement
                     draft-nurpmeso-smtp-tls-srv-04

Abstract

   This document specifies a DNS (RFC 1035) service name in order to
   enable SRV (RFC 2782) records announce the availability of TLS (RFC
   9325) enabled Secure SMTP (RFC 3207, RFC 5321), including Implicit
   TLS.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 7 August 2025.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  SMTP/TLS SRV Service Name . . . . . . . . . . . . . . . . . .   3
   3.  Example: service record . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Rationale  . . . . . . . . . . . . . . . . . . . . .   7
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   [RFC2782] defines a widely adopted DNS-based service discovery
   protocol.  [RFC6186] is a specification of SRV[RFC2782] records for
   the email protocols IMAP[RFC9051], POP3[RFC1939], and
   SUBMISSION[RFC6409].  This includes DNS service names for Implicit
   TLS protocol variants.  SMTP[RFC5321] connections to MTAs ([RFC5598])
   are yet excluded from their share on the widely supported SRV record,
   at least from the IETF point of view.  (According usage however
   exists for many years, see for example the German De-Mail definition
   law; and in recent years notable Open Source MTA implementations
   gained support for SRV lookups.)  Additionally no Implicit TLS SMTP
   protocol variant has ever been specified by the IETF, even though the
   achievable packet roundtrip savings for the SMTP protocol are
   immense, and despite the fact that many (most open source) SMTP
   software implementations already have, or can easily implement,
   built-in support for this mode.  This specification adds a SMTP/TLS
   service name for SRV[RFC2782] records.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The term "Implicit TLS" refers to the automatic negotiation of TLS
   whenever a TCP connection is made on a particular TCP port that is
   used exclusively by that server for TLS connections.  The term
   "Implicit TLS" is intended to contrast with the use of the STARTTLS
   command in SMTP that is used by the client and the server to
   explicitly negotiate TLS on an established cleartext TCP connection.

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2.  SMTP/TLS SRV Service Name

   The service name for TLS[RFC9325] enabled Secure[RFC3207]
   SMTP[RFC5321] is smtps, the resulting DNS label _smtps.

   STARTTLS
      Whenever a domain publishes an according DNS SRV[RFC2782] resource
      record it asserts availability of Secure SMTP, that is, of the
      STARTTLS[RFC3207] SMTP service extension on the normal
      SMTP[RFC5321] port, specified by IANA as port 25.  The port number
      MUST be given as 25.

   Implicit TLS
      If the port number of the published SRV resource record does not
      equal the IANA port 25, then the domain announces support for
      Implicit TLS on the given port in addition to STARTTLS[RFC3207]
      support on the IANA port.  Servers SHOULD NOT announce STARTTLS in
      the EHLO command response of an Implicit TLS connection.  If a
      client issues a STARTTLS[RFC3207] extension command during an
      Implicit TLS connection, the SMTP reply code 554 SHOULD be
      returned, if enhanced status codes[RFC3463] are used, 5.5.1 SHOULD
      be used.

   DNS SRV RRs MUST take priority over MX[RFC5321] ones: no MX lookup
   SHOULD be performed, and no otherwise available MX RR MUST be used in
   the presence of a SRV RR.  After a successful _smtps DNS SRV lookup
   clients SHOULD NOT fallback to cleartext communication.  Servers and
   clients which make use of the _smtps DNS SRV SHOULD follow the
   guidelines of TLS[RFC9325].

3.  Example: service record

   Here are two examples, the first announcing to support the STARTTLS
   SMTP service extension, the second specifying the port number 26 to
   indicate an additional Implicit TLS port.

   _smtps._tcp     SRV 0 1 25 mail1.example.com.

   _smtps._tcp     SRV 0 1 26 mail2.example.com.

4.  IANA Considerations

   The author does not expect IANA to add a dedicated port for "Implicit
   TLS SMTP", given that it has registered according ones for other
   email protocols, for example POP3S, no sooner but 1999, a quarter of
   a century ago.  The author nonetheless asks for assigning a dedicated
   port for Implicit TLS SMTP, and suggests port 26 (which, at the time
   of this writing, was already successfully used for that purpose) in

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   the privileged port range.  The author wants to point out that the
   contra arguments given in section 7 of [RFC2595] that created
   according POP3S and IMAPS assignments in 1999 are contradicted by
   operational reality in the internet, and here that includes the IETF
   by means of [RFC8314].  A dedicated port enables administrators to
   apply strict policies, for example in firewalls.

5.  Security Considerations

   This specification avoids downgrade attacks on the opportunistic
   approach of STARTTLS, accomplished via the mechanism used for many
   other IETF standardized protocols, most notably [RFC2782] (IMAP,
   POP3, SUBMISSION).  With its Implicit TLS capability it grants the
   SMTP protocol the same level of confidentiality through TLS[RFC9325]
   as is already standardized for the other email protocols; this is
   considered a value by itself, even for the possibly lesser sensitive
   MTA to MTA communication.  Implicit TLS reduces the number of packet
   roundtrips, that at a protocol stage where the widely used command
   pipelining[RFC2920] performance improvement extension cannot be used;
   it must be said that today these roundtrips are anachronistic (also
   environmentally): for example the about 4.2 million known DANE for
   SMTP[RFC7672] enabled domains at the time of this writing alone,
   which must use TLS by standard definition, likely generate (several)
   billion(s) of useless sequential and blocking roundtrip packets each
   and every day.  The security of DNS[RFC1035] is out of scope for this
   specification, but DNSSEC[RFC4033][RFC4034][RFC4035] and secure DNS
   transport[RFC7858][RFC8094][RFC8310][RFC8484][RFC9250] etc exists.
   Selection of the appropriate transport layer security protocol is out
   of scope for this specification, please see for example TLS[RFC9325].

6.  References

6.1.  Normative References

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <https://www.rfc-editor.org/info/rfc3207>.

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
              RFC 3463, DOI 10.17487/RFC3463, January 2003,
              <https://www.rfc-editor.org/info/rfc3463>.

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   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

6.2.  Informative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
              <https://www.rfc-editor.org/info/rfc1939>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, DOI 10.17487/RFC2595, June 1999,
              <https://www.rfc-editor.org/info/rfc2595>.

   [RFC2920]  Freed, N., "SMTP Service Extension for Command
              Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920,
              September 2000, <https://www.rfc-editor.org/info/rfc2920>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <https://www.rfc-editor.org/info/rfc4034>.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              DOI 10.17487/RFC5598, July 2009,
              <https://www.rfc-editor.org/info/rfc5598>.

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   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186,
              DOI 10.17487/RFC6186, March 2011,
              <https://www.rfc-editor.org/info/rfc6186>.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
              <https://www.rfc-editor.org/info/rfc6409>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [RFC7672]  Dukhovni, V. and W. Hardaker, "SMTP Security via
              Opportunistic DNS-Based Authentication of Named Entities
              (DANE) Transport Layer Security (TLS)", RFC 7672,
              DOI 10.17487/RFC7672, October 2015,
              <https://www.rfc-editor.org/info/rfc7672>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
              Transport Layer Security (DTLS)", RFC 8094,
              DOI 10.17487/RFC8094, February 2017,
              <https://www.rfc-editor.org/info/rfc8094>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
              for DNS over TLS and DNS over DTLS", RFC 8310,
              DOI 10.17487/RFC8310, March 2018,
              <https://www.rfc-editor.org/info/rfc8310>.

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   [RFC8314]  Moore, K. and C. Newman, "Cleartext Considered Obsolete:
              Use of Transport Layer Security (TLS) for Email Submission
              and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,
              <https://www.rfc-editor.org/info/rfc8314>.

   [RFC8461]  Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
              and J. Jones, "SMTP MTA Strict Transport Security (MTA-
              STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
              <https://www.rfc-editor.org/info/rfc8461>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [RFC9051]  Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message
              Access Protocol (IMAP) - Version 4rev2", RFC 9051,
              DOI 10.17487/RFC9051, August 2021,
              <https://www.rfc-editor.org/info/rfc9051>.

   [RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over
              Dedicated QUIC Connections", RFC 9250,
              DOI 10.17487/RFC9250, May 2022,
              <https://www.rfc-editor.org/info/rfc9250>.

   [RFC9325]  Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/info/rfc9325>.

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/info/rfc9460>.

Appendix A.  Rationale

   Plenty of methods have been standardized by the IETF to perform
   service TLS capability discovery for SMTP.  They have in common that
   they represent SMTP specific solutions to a problem that other
   protocols address by means this document thus reiterates for SMTP.
   They represent further development effort and error surfaces.  They
   also impose increased permanent administration effort, and, due to
   this, are inaccessible to a large number, if not the majority, of
   private email server operators: either through additional costs, the
   impossibility to set up a dedicated name server, if so required, and,
   also as a superset of the former, restrictive web interfaces that
   support only a minimal set of DNS[RFC1035] resource record types.

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   Regardless the fact that several of these extensions switch the SMTP
   protocol to a MUST usage of TLS, they still require the additional
   STARTTLS plus HELO roundtrips that for example [RFC8314] disregards;
   and even though that RFC explicitly refers to DANE for SMTP[RFC7672],
   giving no reason for why SMTP "requires a different approach", except
   for the TLS certificate discovery that could and should be hoped for
   as a general solution, including for clients for any sort of network
   protocol, DANE falls in this category, too: what this specification
   tries to achieve is therefore not counteracting [RFC7672], but
   accompanying it; the author wants to state again that according
   mechanisms already exist in active use, and that many SMTP client/
   server software implementations already support SRV RR lookups:
   activating this existing code base by standard means is only a
   consequence.

   The other extension that [RFC8314] mentions, MTA-STS[RFC8461],
   requires SMTP operators to install and maintain an additional HTTP
   protocol server, accessible on the usual HTTP/S ports and the same
   domain as the email service.  Dependent on the software this may
   require non-trivial and error prone path access policy server rules.
   It is an approach that is complicated to implement, and that brings
   the complexity of another network protocol in the SMTP path; given
   that several different HTTP protocols exist, which in turn base upon
   several different transport protocols, the complexity and protocol is
   alien to SMTP.  The SVCB[RFC9460] DNS approach would not bring
   additional value to the problem in question, but on the other hand
   requires additional implementation efforts as email protocols do not
   make any use of it as of today.

   Coming back to DANE for SMTP[RFC7672] it must be said that
   unfortunately DNSSEC[RFC4033][RFC4034][RFC4035] is still consciously
   unsupported by major players as of today.  Furthermore provider web
   interfaces do often not allow users creation of the necessary DANE
   TLSA resource records: not always can the answer be "run your own
   name server", also letting aside the administrative effort of doing
   so.  However, discovering an Implicit TLS enabled email service via
   the _smtps._tcp SRV record, thereafter checking for applicability of
   DNS-Based Authentication of Named Entities (DANE)[RFC6698], possibly
   even Using Raw Public Keys in Transport Layer Security (TLS)[RFC7250]
   would be a bright future that addresses possibly all of today's
   issues.

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Appendix B.  Acknowledgements

   Thanks to Jan Ingvoldstad for remarks on the missing rationale.
   Thanks to Jeremy Harris for spending time and revealing the many
   problems of early draft variants, we well as comments on how to do it
   better; very special thanks to him for a kickstart implementation of
   this very draft in the widely used exim Open Source MTA.  Jeremy
   Harris, Viktor Dukhovni and Wietse Venema commented on the initial
   odd usage of port 0 for the STARTTLS discovery case.  Thanks to Jonas
   Stalder for corrections and further assistance.

Author's Address

   Steffen Nurpmeso (editor)
   Email: steffen@sdaoden.eu

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